Various types of projection type displays are currently used as image display devices for consumer TV, presentation, industry, large theater, and the like. In general, light is emitted to a spatial light modulating means called a light valve (optical valve) to form an image and then light passing therethrough or light reflected thereon is projected to an auxiliary surface such as a screen to display the image.
In a generally employed structure of the projection type displays, an optical system for lighting the light valve with light emitted from a light source is called a lighting optical system and an optical system for enlarging and projecting a light beam emitted from the light valve is called a projection optical system.
The lighting optical system for the projection type displays requires a function for uniformly lighting the light valve and a function for efficiently lighting the light valve. A spatial intensity distribution of a light beam from a lamp light source is non-uniform. When illumination unevenness occurs on the light valve, a variation in brightness occurs on a projected image. Therefore, a mixing rod (or rod integrator or light tunnel) type optical integrator is used for a method of uniformly lighting the light valve (see, for example, Patent Document 1).
According to this type, a light beam from a light source is condensed by a lens and allowed to enter an incident port of the mixing rod. Then, the entered light beam is repeatedly reflected in the rod and superimposed, so an exit end surface of the rod can be assumed to be a secondary light source having a uniform distribution.
Because of the above-mentioned reason, a numerical aperture (NA) of light incident on the mixing rod is held at a numerical aperture of light exited from the mixing rod. The exited light beam is uniformly emitted without unevenness to the light valve serving as an illuminated surface by a relay optical system (lighting optical system) including a lens and a mirror. The optical integrator has a beam shaping action as another effect thereof. When the light beam having a substantially circular cross sectional shape from the lamp light source is converted into light having a square shape substantially identical to that of the light valve, light use efficiency can be improved.
The exit end surface of the mixing rod is set as the light source and an image from the light source (exit end surface) is formed on the light valve by the relay optical system. Here, assume that the numerical aperture NA of the light exited from the mixing rod is expressed by Nr, an aperture size of the mixing rod is expressed by Wr, the numerical aperture NA of illumination light is expressed by Ni, and an aperture size of the light valve is expressed by Wi. Then, a lateral magnification β of the lighting system is expressed by β=Wi/Wr. When the lighting system satisfies the Abbe sine condition, β=Nr/Ni, therefore, Nr×Wr=Ni×Wi, so a product of the numerical aperture and the light source size is maintained. This may be assumed as the Lagrange-Helmholtz invariant in a paraxial region. In actuality, the equal sign is not necessarily satisfied because of lens aberration and eclipse.
Even when the light valve is uniformly and efficiently lighted, the uniform and efficient lighting should be reflected to the screen by the projection optical system. Therefore, it is necessary to match the numerical aperture of the illumination light lighting the light valve with the numerical aperture of the projection optical system. This is because, when the numerical aperture NA of the illumination light is larger than the numerical aperture NA of the projection optical system, illumination light corresponding to a difference therebetween is not incident on the projection optical system, thereby reducing the light use efficiency. Thus, a set of the projection optical system and the lighting optical system and a set of the lighting optical system and the light source cannot be freely selected in numerical aperture and size.
When the lamp light source for the projection type displays is replaced by a laser light source, the widening of color reproduction range obtained by a monochromatic spectrum, an increase in contrast made by light source intensity modulation, the lengthening of life which is caused by using a semiconductor laser or the like, and a reduction in size of an optical system in view of light directivity can be realized. Therefore, performance which is significantly higher than that of the conventional lamp light source can be obtained.
A product of the numerical aperture NA and a beam size in the laser light source becomes very smaller than that in the lamp light source. Therefore, when the numerical aperture NA of the illumination light and the aperture size of the light valve are determined, the numerical aperture NA of the light incident on the mixing rod becomes smaller or the beam size of the incident light becomes smaller. When the numerical aperture NA is small, the number of repetition of total reflection in the rod becomes smaller. When the beam size is small, a bias of a spatial intensity distribution becomes larger in the aperture of the rod. Thus, it is difficult to uniformly light the light valve by the mixing rod used for the conventional lamp light source.
An example of the method of uniformly lighting the light valve includes a method of coupling light from the laser light source to a multimode optical fiber to emit uniformly distributed light exited therefrom to the light valve (see, for example, Patent Document 2).
However, the optical fiber has a circular cross sectional shape and the light exited therefrom becomes a light beam having a substantially circular shape. Therefore, the shape of the light beam is different from that of the light valve, so the light use efficiency reduces.
Laser light has a single wavelength. In order to obtain a wide color reproduction range, laser beams having a plurality of wavelengths corresponding to three primary colors of red (R), green (G), blue (B) are required. Therefore, when the laser beams having the plurality of wavelengths from a plurality of light sources are to be combined with one another and emitted to a single light valve, an optical system for combining the laser beams is complicated, so the apparatus becomes large and expensive.
When laser light is emitted to the light valve and projected to the screen, a light and dark spot pattern which is called speckle appears on an image. This is because, the laser light which is spatially and temporally coherent light is reflected on an optically rough surface or passes therethrough to become light having a different phase, so the light having the different phase is observed as interference light by human's eyes. The speckle becomes an image noise component and thus this is not preferable for an observer.
An example of a method of removing the speckle includes a method of coupling light from a laser light source to a multimode optical fiber and vibrating the optical fiber to cause a mode scramble (see, for example, Patent Document 3).
According to this method, speckle patterns change. When the speckle patterns are superimposed on one another, speckles are averaged and reduced. However, a mechanical apparatus for vibrating the optical fiber is required. The durability of the mechanical apparatus and the optical fiber to be vibrated become a problem. The optical fiber having the circular cross sectional shape generates a skew component along the circumferential direction. Therefore, a spatial bias of the speckle pattern is large, so the speckles cannot be sufficiently removed.
Patent Document 1: U.S. Pat. No. 5,634,704 B
Patent Document 2: JP 2000-121836 A
Patent Document 3: U.S. Pat. No. 3,588,217 B